1. Field of the Invention
The present invention relates to spherical silica composite oxide particles having a high degree of monodispersion property comprising silica and a metal oxide other than silica. More specifically, the invention relates to novel silica composite oxide particles containing a metal oxide other than silica in an amount in a range of from 30 to 50 mol %, that could not be produced thus far.
2. Description of the Related Art
Spherical silica particles having a high degree of monodispersion property can be produced by using, as a starting material, a metal alkoxide such as tetraethoxy silane or tetramethoxy silane, and hydrolyzing and condensing the metal alkoxide in a water-containing organic solvent containing a catalyst. It is also allowable to produce so-called silica composite oxide particles such as of silica-titania, silica-alumina or silica-zirconia by using the above alkoxide of silicon and an alkoxide of a metal other than silicon as starting materials (hereinafter, the above methods of production are called sol-gel methods).
The silica composite oxide particles produced by the sol-gel method is capable of exhibiting a variety of characteristic properties that could not be obtained by using the silica only as a result of forming a composite of various kinds of metal oxides with silica as a main component. By varying the blending ratio of silica and a metal oxide other than silica, for example, the refractive index of particles can be freely adjusted while maintaining optical transparency, which is an excellent feature that could not be observed otherwise.
The silica composite oxide particles of which the refractive index is adjusted as described above are very useful as a filler when it is attempted to obtain a composite thereof with a resin. For example, Japanese Examined Patent Publication (Kokoku) No. 33721/2001 discloses a dental composite resin featuring improved abrasion resistance, surface luster and aesthetic appearance (transparency) by blending a polymerizable vinyl monomer with the above silica composite oxide particles. Further, Japanese Unexamined Patent Publications (Kokai) Nos. 65475/2004 and 41544/2005 disclose transparent composite resins suppressing the coefficient of thermal expansion by obtaining composites thereof with an epoxy resin.
In the above conventional examples of application, however, a sufficiently high refractive index could not be obtained when the content of a metal oxide other than silica was not larger than 20 mol %, since the refractive index of the resin component that is to be blended with the silica composite oxide particles was not substantially larger than 1.57. According to the conventional methods, further, when it is attempted to synthesize a component by containing the metal oxide other than silica in an amount in excess of 20 mol %, the particles coagulate together or fine particles are newly formed during the synthesis, making it difficult to obtain spherical silica composite oxide particles having a favorable monodispersion property.
Japanese Examined Patent Publication (Kokoku) No. 33721/2001 teaches that it is desired to suppress the constituent ratio of a metal oxide other than silica to be generally not larger than 30 mol % and, particularly, not larger than 20 mol % when it is intended to obtain a spherical inorganic oxide, and that excellently spherical particles having uniform sizes are obtained when the constituent ratio of a metal oxide other than the silica is selected to lie in a range of from 0.01 to 15 mol %. Besides, working examples of the above known literatures teach that the content of the metal oxide other than the silica in the silica composite oxide particles is 24 mol % at the greatest.
The present invention provides silica composite oxide particles having a high degree of monodispersion property containing a metal oxide other than silica in an amount of not smaller than 30 mol %, and a method of producing the same.
The present inventors have forwarded keen study in an effort to solve the above problems. As a result, the inventors have discovered that silica composite oxide particles having a high degree of monodispersion property can be synthesized even by using a metal oxide other than silica in an amount of not smaller than 30 mol % by conducting the partial hydrolysis of an alkoxide of silicon under optimum conditions in a very narrow range in preparing a composite alkoxide which is a starting material, i.e., in preparing the composite alkoxide comprising silicon and an alkoxide of a metal other than silicon, and have completed the present invention.
According to the present invention, there is provided silica composite oxide particles comprising silica and a metal oxide other than silica, the content of the metal oxide other than silica being from 30 to 50 mol %, and a coefficient of variation in the particle diameter being not larger than 30%.
According to the present invention, there is further provided a method of producing silica composite oxide particles by partially hydrolyzing a silicon alkoxide with water followed by mixing with an alkoxide of a metal other than silicon to prepare an alkoxide composition, and hydrolyzing and condensing said alkoxide composition in a water-containing organic solvent containing a catalyst for hydrolysis, wherein the partial hydrolysis of the silicon alkoxide is conducted the alkoxide of a metal other than silicon is used so as to satisfy a condition represented by the following formula (1),
xe2x88x920.06X+3.5 less than Y less than xe2x88x920.06X+4.5xe2x80x83xe2x80x83(1) 
wherein X is a ratio (mol %) of the amount of the alkoxide of a metal other than silicon to the amount of the whole alkoxides and is a number of from 30 to 50, and Y is the amount of water used for the partial hydrolysis of the silicon alkoxide and is an equivalent of the water to the alkoxide of the metal other than silicon.